1. Field of the Invention
This invention relates generally to magnetic recording disk drives for use with patterned media, wherein each data bit is stored in a magnetically isolated block or island on the disk, and more particularly to such a disk drive with an improved clock for writing the data.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media have been proposed to increase the data density. In patterned media, the magnetic material on the disk is patterned into small isolated data blocks or islands arranged in concentric data tracks. Each island contains a single magnetic “bit” and is separated from neighboring islands by a nonmagnetic region. This is in contrast to conventional continuous media wherein a single “bit” is composed of multiple weakly-coupled neighboring magnetic grains that form a single magnetic domain and the bits are physically adjacent to one another. Patterned-media disks may be longitudinal magnetic recording disks, wherein the magnetization directions are parallel to or in the plane of the recording layer, or perpendicular magnetic recording disks, wherein the magnetization directions are perpendicular to or out-of-the-plane of the recording layer. To produce the required magnetic isolation of the patterned blocks, the magnetic moment of the regions between the blocks must be destroyed or substantially reduced so as to render these regions essentially nonmagnetic. Alternatively, the media may be fabricated so that that there is no magnetic material in the regions between the blocks.
In one type of patterned media, the data islands are elevated, spaced-apart pillars that extend above the disk substrate surface to define troughs or trenches on the substrate surface between the pillars. This type of patterned media is of interest because substrates with the pre-etched pattern of pillars and trenches can be produced with relatively low-cost, high volume processes such as nanoimprint lithography using a master template created by e-beam lithography and self-assembly, along with an appropriate etching method to transfer the pattern to the substrate. The magnetic recording layer material is then deposited over the entire surface of the pre-etched substrate to cover both the ends of the pillars and the trenches. The trenches are recessed far enough from the read/write head to not adversely affect reading or writing. This type of patterned media is described by Moritz et al., “Patterned Media Made From Pre-Etched Wafers: A Promising Route Toward Ultrahigh-Density Magnetic Recording”, IEEE Transactions on Magnetics, Vol. 38, No. 4, July 2002, pp. 1731-1736.
In patterned media, because the data islands are single-domain, the transitions between bits occur only between the islands. Since the magnetic transitions are restricted to predetermined locations governed by the locations of individual data islands, it is necessary to synchronize the switching of the write current and thus the reversal of the write field from the write head with the passing of individual data islands past the write head. The optimal phase of the write clock that controls the reversal of the write field depends on the position of the write head with respect to the centerline of the data track. This is because the outer boundary or contour of the write “bubble”, i.e., the locus of magnetic field strength sufficient to magnetize the data islands, is curved at its trailing edge. Thus if the write head is not well-centered on the data track, but shifted by an amount of track misregistration (TMR) away from the track centerline, the data islands will not be located within the write bubble when the write field is reversed. This may lead to write errors on the selected track.
What is needed is a magnetic recording disk drive with patterned media that has a write-clock with phase adjustment to compensate for TMR of the write head.